On the importance of E‐field variability for Joule heating in the high‐latitude thermosphere

TLDR

Joule heating is a major energy source in the upper atmosphere, yet its magnitude remains a key uncertainty because conventional models use average convection patterns that ignore rapid E‑field fluctuations. This paper evaluates theoretically how E‑field variability influences Joule heating as an upper‑atmosphere energy source. The authors argue that computing Joule heating requires averaging the square of the E‑field, which demands knowledge of its statistical variability—a capability currently lacking in models that rely on mean convection patterns. They demonstrate that incorporating E‑field variability into high‑latitude convection models can substantially increase calculated Joule heating for a given pattern.

Abstract

Joule heating is known to be one of the major energy sources of the upper atmosphere. Knowledge of the magnitude of this source is fundamentally important to a thorough understanding of the region's physics. However, Joule heating is currently one of the largest sources of uncertainty in the thermosphere's energy budget. In numerical models the distribution of Joule heating is generally computed using mean or average convection patterns, which evolve on a relatively long time scale in response to changes in solar wind conditions. The convection patterns represent average electric potential distributions, and thus the resulting amount of Joule heating is proportional to the square of the average E‐field. That method ignores the important component of Joule heating due to rapid or small‐scale fluctuations in E‐field or ion drifts. However, E‐field fluctuations are known to exist on a variety of temporal and spatial scales, and the actual amount of Joule heating in the thermosphere is proportional to the average of the square of the E‐field. The computation of the average of the square of the E‐field requires knowledge of the statistical characteristics of E‐field variability; thus knowledge not available at present. In this paper we assess, on the bases of theoretical considerations, the importance of E‐field variability as an upper‐atmosphere energy source. We show that the inclusion of E‐field variability in the high‐latitude convection model can significantly increase the amount of Joule heating for a given pattern.